(1) Field of the Invention
The present invention relates to a novel method for preparing cellobiose. The cellobiose is a disaccharide which consists of two molecules of glucose connected by way of .beta.-1,4 linkage and it is known as a smallest constituent unit of cellulose. This cellobiose is more stable as compared with other disaccharides such as sucrose and maltose. In addition, in view of the uses for foods, it is expected to use the cellobiose, for example, as a filler for synthetic sweetening agents because it is non-calorific and low sweet-tasting.
(2) Description of Prior Art
As the method to prepare cellobiose, only methods using the starting material of cellulose have hitherto been known. The methods are exemplified by acidolysis of cellulose and enzymatic decomposition of cellulose. In the former method, cellulose is subjected to acetolysis to produce cellobioseoctaacetate and it is then converted into cellobiose by deacetylation (A. N. Pereira at al., Methods Enzymol., 160, 26 (1988)). In the latter method, cellobiose is directly produced from cellulose by treating cellulose with a cellulose-hydrolyzing enzyme of cellulase (Hajime Taniguchi, Nippon Nogeikagaku Kaishi, 63, 1133 (1989)).
In both the foregoing methods, cellobiose is prepared as a decomposition product of cellulose. Incidentally, cellulose is contained as the main component of cell walls of plants, however, it does not exist singly but it exists in the form of mixtures with hemicellulose, lignin and so forth. Accordingly, when cellulose is used as a starting material in the foregoing conventional methods, it is necessary that the hemicellulose and lignin must be removed beforehand from the cellulosic starting material by a suitable method such as alkali treatment, which fact causes to raise the cost of starting material. In addition, because a strong acid is used for the reaction in the acidolysis method, the application to foods is difficult in view of hygienic or safety problem. Meanwhile, there is a problem in the method of enzymatic decomposition that no cellulase which is sufficiently active to cellulose can be obtained. Owing to these reasons, there is proposed at present none of suitable method to produce a large quantity of cellobiose at low cost.